An increasing world population leads to a continually increasing amount of refuse. Additionally, an increased level of civilization appears to generate an increased amount of refuse on a per capita basis. Both factors in combination lead to mounting pressure to devise methods of waste disposal which are economically, energetically, and environmentally sound.
In recent years, especially in urban areas, the increased demand for usable land and other concerns has caused one to turn from a landfill as the major mode of refuse disposal to other options, especially the use of raw refuse as an energy source. One variant of the latter is the mass burning approach, where all the refuse in its raw state is burned without any preliminary treatment such as separating the noncombustible from combustible material. Quite briefly, in this method raw garbage is dumped into storage where it is homogenized and dried to some degree. Refuse from the storage area is fed into a combustion zone where the heated gases often are used to generate steam. Flue gases then pass from the combustion zone to a separation zone, often an electrostatic precipitator, where dust and ash are removed. The ash so removed from the flue gas, called fly ash, is then mixed with the ash collected in the combustion zone, called bottom ash, and the combined ash used for landfill, in road construction, and so forth.
It is well known that some of the more volatile compounds of certain metals tend to accumulate in the fly ash. Especially where the latter is to be used as landfill, leaching of toxic metals, especially cadmium and lead, constitutes a potential hazard to the ecosystem, for example, both surface water supplies and aquifers. The Environmental Protection Agency (EPA) has promulgated a procedure to determine the toxicity of solid wastes, and where residues exceed the toxicity as stated in the Federal Register Code 40, No. 26124, the waste is classified as a hazardous waste requiring control under the Hazardous Waste Management System. A recent report prepared for the Office of Solid Waste, U.S. Environmental Protection Agency, which was a limited survey of several kinds of solid waste, seems to suggest that levels of cadmium and lead in fly ash pose perhaps the most serious environmental threat, and that such fly ash alone would need to be treated as a hazardous waste; EP Toxicity Test Results on Residues from Eight Resource Recovery Facilities, SYSTECH Corporation, February, 1981.
The environmental hazard of fly ash containing amounts of cadmium and lead greater than the toxic levels specified by the EPA is somewhat diminished by mixing such ash with heavy ash, such that the resulting landfill mixture is within the toxic levels for the cited metals. Nonetheless, it is highly desirable to reduce the amount of cadmium and lead leached from fly ash and other solid waste to an amount below the toxic levels specified by the EPA. The invention herein is a solution to this problem. More specifically it is a method of treating dry, solid residues, especially fly ash, and mixtures containing fly ash, so as to reduce the amounts of cadmium and lead leached from such residues to a level below the toxic level specified by the EPA. Stated differently, the invention herein is a method of immobilizing, or insolubilizing, cadmium and lead in solid waste, especially over a wide pH range. The method is convenient, quite simple, very efficient, applicable over a wide pH range, and relatively low cost. The method is, therefore, commercially extraordinarily attractive as well as being environmentally beneficial.
The problem we have addressed is not new; only our solution to this problem is new. Prior solutions have relied on transforming metal-laden ash into a solid, hardened, often brick-like consistency to immobilize lead and cadmium. Such solutions are based on producing a product largely impermeable to water, thereby reducing, if not eliminating, metal transport by diffusion. In contrast, our invention retains the powdery (particulate) nature of the ash-containing residues while immobilizing lead and cadmium; the treated residue remains a particulate, non-hardened solid which does not harden to a brick-like consistency and this characteristic serves as a distinguishing feature of our invention.
The precipitation of heavy metals, including cadmium and lead, at high pH is a well-known analytical technique, and the use of lime as the basic agent is a common procedure. For example, solid wastes containing cadmium and lead were treated with 3-15% calcium hydroxide and/or magnesium sulfate, the pH was adjusted to 8-10.5, and the solid coated with asphalt to prevent the leaching of cadmium and lead. Chemical Abstracts, 92; 185414d. The preceding method is a mixture of coagulation-flocculation followed by encapsulation in a hydrophobic, petroleum-based solid.
In U.S. Pat. No. 4,049,462 Cocozzo treated industrial desulfurization residues resulting from removal of sulfur oxides from effluent gas with alkaline calcination stack dust and water under acidic conditions to form a solid, hardened, leach-resistant product. The patentee recognized that the cement-like product resulted from the reaction of calcium oxide and silicate in the stack dust with acid anions, whose nature was not significant so long as the mass reacted under acidic conditions to provide a hardenable mass which upon drying became cementitious solid.
Pichat describes a process to transform strongly acidic liquid wastes containing relatively high metals content, including cadmium, into solid materials by mixing the wastes with coal fly ash, adjusting the pH to about 7, adding a lime-containing substance and a binder, such as Portland cement, with the mixture setting to a petrified mass; U.S. Pat. No. 4,375,986. As the patentee recognized, coal ash is pozzolanic, i.e., in the presence of lime it agglomerates into a hard, compact, mortar type product. Clearly, Pichat's invention describes a method to treat acidic liquid wastes and uses coal fly ash as an additive. The patentee also recognizes that coal fly ash does not contain sufficient amounts of Pb and Cd to present an environmental concern. Although Schneider-Arnoldi et al. in U.S. Pat. No. 3,676,165 teach that phosphorous furnace slag can be substituted for lime as a binder, and that such slag contains phosphorous compounds in the amount of 0.5-2.0% reported as P.sub.2 O.sub.5, the slag is a hard vitreous mass which fails to furnish soluble phosphate, an essential element of our invention. In fact, such slag contains phosphorous chiefly as calcium phosphate, which we show to be inoperative in immobilizing lead and cadmium.
In all instances reported in which a cement-like material is fabricated from fly ash, the inventors use coal fly ash which due to chemical composition, surface composition and morphology, and size distribution is pozzolanic. However, for these same reasons, incinerator fly ash is not pozzolanic and cannot form a stable cement in the absence of ordinary portland cement. The invention described here does not require ordinary portland cement and neither requires nor utilizes solidification or agglomeration for its successful application. Methods applicable to agglomeration or fixation of coal fly ashes are simply not readily applicable to incinerator fly ashes.
A base course for pavement construction can be made from incinerator ash reacted with lime and water prior to compaction; U.S. Pat. No. 4,496,267, European Pat. No. 34-389, directed toward the agglomeration of coal fly ash into pellets, discloses some phosphorous compounds in the ash and reports the total phosphorous content as P.sub.2 O.sub.5, but as with Schneider-Arnoldi et al. this phosphorous source does not furnish soluble phosphates.
We have discovered a method of immobilizing lead and cadmium in refuse-to-energy combustion residues effective over a broad pH range to reduce the leaching of the aforementioned heavy metals to a level below the maximum dictated by the EPA. Quite simply, the method involves treatment of the solid residues with lime followed by addition of a water soluble phosphate. Using this method levels of lead and cadmium are reduced to less than 5 and 1 ppm, respectively. It is also desirable to immobilize the toxic metals to pass the regulatory limits with a typical acid rain or water extraction. This requires an immobilization system which is effective over the entire pH range above about 5.0; the method we have discovered meets this requirement. Our method does not change the particulate nature of the untreated solid residue; it generates no cement-like mass. Our method does not generate calcium phosphate as the metal binder; substitution of calcium phosphate for our soluble phosphate fails to immobilize lead and cadmium. Whatever may be the detailed mechanism of metals immobilization in our method, it appears that our immobilizing materials of lime and soluble phosphate remain quiescent and inactive in the dry solid residue, but when water--the extractant--perfuses through the solid the immobilizers raise a barrier to dissolution and/or diffusion of the metals into the liquid phase.